1. Field of the Invention
The present invention relates to a die usable for forming a honeycomb structure of high isostatic strength and a high precision, and being comprised of a plurality of cells defined by partition walls, and a manufacturing method of the die. More particularly, it relates to a honeycomb structure forming die usable for forming a honeycomb structure having a high isostatic strength and being comprised of cells having a pentangular shape or a more polygonal shape such as a hexagonal shape, a combination of cells having different sectional shapes, cells having different dimensions of from large to small cells in dimension or the like and in which partition walls of the cells on the outer peripheral side of the honeycomb structure are formed to be thicker than those of the cells in the center thereof, and a manufacturing method of the die.
2. Description of the Related Art
Heretofore, as a catalyst for purifying an automobile exhaust gas, a so-called honeycomb catalyst has been formed in which catalyst components are loaded on the surfaces of cells of a ceramic honeycomb carrier (a honeycomb structure), and such a structure that the honeycomb carrier has been held along an axial direction thereof because of a higher strength of the structure along the axial direction than that along its sectional (diametric) direction. In this case, to prevent the breakdown of the honeycomb carrier having been held along the axial direction thereof around the outer peripheral portion thereof, the cell partition walls (ribs) of the outer peripheral portion are formed to be thicker than inner partition walls, whereby the pressure withstanding strength of the honeycomb carrier along the axial direction is increased.
However, in recent years, because of demand for the decrease of the pressure loss of the exhaust gas in the honeycomb catalyst involving the orientation for the high output of an engine or demand for the effective utilization of the whole catalyst carrier to cope with the tightening of regulations on the exhaust gas, instead of the structure wherein the honeycomb catalyst carrier is held along the axial direction, a structure wherein the outer peripheral surface of the honeycomb catalyst carrier has been mainly held has started to be employed. One of reasons for the employment of the structure is that the tightening of the regulations on the exhaust gas causes the increase of the volume of the catalyst and the increase of the mass of the catalyst, and the structure wherein the catalyst carrier is held along the axial direction has such a small holding area that the catalyst structure cannot sufficiently be held against the vibration of the engine.
On the other hand, to improve the purification performance of the catalyst, there has been started a movement for decreasing the thicknesses of the cell partition walls of the honeycomb carrier to decrease the weight of the honeycomb carrier, thereby decreasing the heat capacity of the catalyst to improve warm-up characteristics in the purification performance. Furthermore, the decrease of the thicknesses of the walls also noticeably contributes to the decrease of the pressure loss.
Consequently, owing to the decrease of the thicknesses of the cell partition walls, the breakdown strength of the honeycomb carrier against an external pressure applied from the outer peripheral surface thereof tends to further lower. Furthermore, owing to the recent tightening of the regulations on the exhaust gas, the improvement of the burning conditions of the engine and the improvement of the purification performance of the catalyst are aimed, and the temperature of the exhaust gas rises yearly, whereby a resistance to thermal shock is strongly required for the honeycomb carrier. In this way, the thicknesses of the cell partition walls are decreased, the structure wherein the outer peripheral surface of the honeycomb carrier is held is employed, and resultantly the temperature of the exhaust gas rises. For these and other reasons, the setting of the thicknesses of the cell partition walls and a honeycomb outer wall, the increase of the isostatic strength of the honeycomb structure and the increase of the precision of an outer shape or a partition wall shape have become major themes.
As the honeycomb carrier having high isostatic strength and a highly precise structure shape, there is proposed a carrier having a structure in which only an outer peripheral rib is thickened and strengthened as compared with an inner peripheral rib. A die for extrusion-forming the honeycomb carrier is provided with a structure in which an outer peripheral slit is formed, by grinding, into a large slit as compared with an inner peripheral slit so as to increase the thickness of the outer peripheral rib as compared with the inner peripheral rib (see Patent Document 1).
As a manufacturing method of a ceramic honeycomb structure having hexagonal lattice-like cells, the combination of cells having different sectional shapes or cells having different dimensions of from large to small cells in dimension, heretofore an extrusion-forming method using such a die for forming the honeycomb structure that a die includes a die base member provided with back holes (hereinafter referred to as the introduction hole sometimes) through which a ceramic forming material (hereinafter referred to as the clay sometimes) is introduced, and hexagonal lattice-like slits connected to the back holes has been known. For example, the hexagonal lattice-like die is usually provided with hexagonal slits having widths corresponding to the thicknesses of partition walls of the honeycomb structure in one end face of the die base member, and the back holes connected to the slits and having large opening areas in the opposite end face thereof (the other end face). Moreover, the back holes are usually provided at positions where hexagonal slits intersect with one another, and are connected to the slits in the die base member. Therefore, a forming material such as a ceramic material introduced through the back holes is moved from the back holes having comparatively large inner diameters to the slits having small widths, and is extruded through the open frontal areas of the slits to form a formed article of the honeycomb structure (the formed honeycomb article).
As the manufacturing method of the die for forming the honeycomb structure having such hexagonal lattice-like cells, a manufacturing method is disclosed in which the above slits having a honeycomb shape are formed by electric discharge machining (EDM) (e.g., see Patent Documents 2, 3).
According to the manufacturing method of a honeycomb die disclosed in Patent Documents 2 and 3, the manufacturing method of the honeycomb die for manufacturing a honeycomb article having hexagonal lattice-like cells is described. The honeycomb structure including such hexagonal lattice-like cells has low shape retention ability during extrusion-forming and easily causes the deformation of the cell lattice as compared with a honeycomb structure having quadrangular cells, whereby it has been difficult to increase the isostatic strength.
Moreover, in a catalyst carrier using a catalytic function in an internal combustion engine, a boiler, a chemical reaction apparatus, a reformer for a fuel cell or the like, a filter for collecting fine particles, especially diesel fine particles in the exhaust gas (hereinafter referred to as the diesel particulate filter (DPF) sometimes) or the like, heretofore, the honeycomb structure made of a ceramic material has been used.
The honeycomb structure having been used for such a purpose usually includes a plurality of cells defined by porous partition walls as through channels for a fluid. Especially, in a case where the honeycomb structure is used as a fine particle collecting filter, the honeycomb structure has a structure in which adjacent cells are plugged at opposite ends alternately so as to form a checkered pattern at both end faces. In the honeycomb structure having such a structure, the fluid to be treated flows into each cell having an inflow side end face which is not plugged, that is, the cell having an outflow side end face plugged, passes through each porous partition wall, and is discharged from the adjacent cell, that is, the cell having the inflow side end face plugged and the outflow side end face which is not plugged. In this case, the partition walls function as the filter. When the honeycomb structure is used as a DPF, a particulate matter (hereinafter referred to as “the PM” sometimes) such as soot discharged from a diesel engine is collected by, and accumulated on the partition walls.
Furthermore, as the honeycomb structure to be used as such a DPF, a plugged honeycomb structure is known in which the sizes of cell open frontal areas in one end face are different from those in the other end face. This plugged honeycomb structure having the sizes of the cell open frontal areas in the one end face which are different from those in the other end face thereof is a plugged honeycomb structure including the combination of the cells having different sectional shapes or the cells having large and small dimensions, and the sizes (areas) of the inflow side cell open frontal areas are set to be larger than those (areas) of the outflow side cell open frontal areas, to improve a PM collecting efficiency. Specific examples of the cells in which the sizes of the cell open frontal areas in the one end face are different from those in the other end face include the combination of quadrangular cells having large and small dimensions, and the combination of quadrangular and octagonal cells.    [Patent Document 1] JP-A-2003-94415    [Patent Document 2] JP Patent No. 1784822 (JP-B-04-74131)    [Patent Document 3] JP Patent No. 1784823 (JP-B-04-74132)
However, in a die for forming the honeycomb structure described in Patent Document 1, the slits are realized by grinding with a disc grindstone, and this manufacturing method cannot be applied to the honeycomb structure including the pentangular lattice-like cells or more polygonal lattice-like cells such as hexagonal lattice-like cells, the plugged honeycomb structure in which the sizes of cell open frontal areas in the one end face are different from those in the other end face or the like. Hereinafter, the method will be described with reference to, for example, a plan view of the other end face of the die base member provided with slits 5 seen from a thickness direction 201 of the die base member. FIGS. 10A to 10D show several pattern examples of the lattice-like slits 5 of the die for forming the honeycomb structure. In FIGS. 10A to 10D, at least a part of a plurality of lattice-like partition regions 3 defined by the slits 5 is provided in such a position that the slits 5 overlap along the extended line of the edges of lattice-like regions in the plan view seen from the thickness direction 201 of the die base member, and hence the die cannot precisely be manufactured by the grinding with the disc grindstone or the like.
In a honeycomb structure forming die including a hexagonal lattice-like slits 5 as shown in FIG. 10A, the slits are provided in such a position that the slits 5 overlaps with the extended line along the edges of hexagonal lattice as shown by a one-dot chain line, and hence the die cannot precisely be manufactured by the grinding with the disc grindstone or the like.
Moreover, in a die for forming a plugged honeycomb structure in which the sizes of the cell open frontal areas in the one end face are different from those in the other end face, the die including lattice-like slits 5 having been composed of different dimensions of large and small sizes as shown in FIG. 10B, or in honeycomb structure forming dies including lattice-like slits having a pentangular shape or a more polygonal shape as shown in FIGS. 10C, 10D, a part of partition regions is provided so as to overlap with the extended line of the slits 5 along the edges of the lattice-like regions as shown by a one-dot chain line, and hence the die cannot precisely be manufactured by the grinding with the disc grindstone or the like.
Furthermore, in Patent Document 1, an enlarging slit portion is processed by the grinding, and the resolution of the depth control of the enlarging slit portion depends on the diameter of the used grindstone. In consequence, an irregularity is made in a slit width between the inner peripheral portion of the die and the outer peripheral portion thereof, and hence a problem occurs that a forming defect such as a cell defect easily occurs. Moreover, another problem occurs that if the discontinuous portion of the slit width occurs in the enlarging slit portion and an ordinary portion even within one slit, the forming defect is easily caused in the discontinuous portion.
The present invention has been developed in view of such problems of the conventional technology, and an object thereof is to provide a honeycomb extrusion-forming die which can impart higher dimensional precision and strength even to a honeycomb structure including pentangular or more polygonal lattice-like cells or cells having large and small sizes, and a manufacturing method of the die.